Generally, a display device is required to have a viewing angle as wide as possible so that a viewer can view displayed images clearly from any viewing point. In order to achieve such a wide viewing angle, various technical development has been undertaken, particularly, a liquid crystal display device, which has been widely used recently. This is because the liquid crystal display device employs liquid crystal which itself has a viewing angle characteristic.
However, in some environments a user wishes to privately look at content displayed on a display screen. In such a case, it is preferable that the viewing angle be narrow. Such may be the case for a user who uses an electronics device, such as a laptop personal computer, a mobile terminal (PDA: Personal Digital Assistant), or a mobile phone, in a place where other people are near the user, such as on a train, or on a plane. In such environments, the user wishes to prevent other people near the user from seeing the displayed content in view of preservation of confidentiality, privacy protection, and the like. Therefore, in those cases it is preferable that the display device has a narrow viewing angle. For this reason, recently there has been growing demand for a display device whose viewing angle can be switched between a wide viewing angle and a narrow viewing angle in accordance with an environment in which the user uses the display device. Not only is this demanded for the liquid crystal display device, but is also demanded for any liquid crystal display device in common.
In response to such demand, the following Patent Literature 1 discloses a technique of changing a viewing angle characteristic of a display device by (i) providing a display device for displaying images with a phase difference control device, and (ii) controlling a voltage applied to the phase difference control device, for example. Patent Literature 1 describes that the liquid crystal display device functioning as the phase difference control device may employ, as the liquid crystal mode, chiral nematic liquid crystal, homogeneous liquid crystal, or nematic liquid crystal in random alignment, for example.
Patent Literature 1 describes that a switchover between the wide viewing angle and the narrow viewing angle can be realized by using the liquid crystal display device for phase difference control. However, the switchover between the wide viewing angle and the narrow viewing angle is not absolutely necessary.
The switchover of the viewing angle requires the display device to have a more complicated arrangement, and also requires an increase in cost. In consideration of a reduction in cost of manufacture of the display device, there has been demand for a display device which does not have a function of the switchover of the viewing angle but is capable of narrowing its viewing angle.
The following Patent Literature 2 discloses a technique of narrowing a viewing angle of a display device which does not have the function of the switchover between the wide viewing angle and the narrow viewing angle. Patent Literature 2 discloses a viewing angel control sheet which takes the place of the phase difference control device having the function of the switchover of the viewing angle. As illustrated in FIG. 23, a louver film 102, which functions as the viewing angle control sheet, is stacked on an upper surface of a liquid crystal panel 101 having a liquid crystal cell 111 containing TN (Twisted Nematic) liquid crystal. As illustrated in FIG. 24, this louver film 102 includes: base material sections 121, which are made of a transparent material; and light absorbing walls 122, which are made of a light absorbing material and are arranged in a louver pattern. The light absorbing walls 122 made of the light absorbing material are lower in refractive index than the base material sections 121 made of the transparent material. The louver film 102 prevents light that enters into the louver film 102 at an angle not less than a specific incident angle from being emitted from the louver film 102. Therefore, it is possible to realize the viewing angle control sheet having an effect of preventing other people from seeing the displayed content.
Recently, this louver film disclosed in Patent Literature 2 has been mainly used in a viewing angle control panel without the switchover of the viewing angle.
The louver film disclosed in Patent Literature 2, however, can limit visibility only in oblique directions, opposite to each other, toward a light receiving surface of the light absorbing wall 122. In other words, the louver film has a problem that it is impossible to arbitrarily set a direction in which the visibility is limited.
Further, the louver film has another problem of high cost due to such a periodical structure that the light transparent material and the light absorbing material are arranged alternatively.
In consideration of the aforementioned problems of the louver film, Patent Literature 3 discloses a viewing angle limiting element which has another arrangement in place of the above louver film.
This viewing angle limiting element has such an arrangement that (i) a liquid crystal layer in which liquid crystal molecules have hybrid alignment is sandwiched between a first polarizing layer and a second polarizing layer, and (ii) an absorption axis of the first polarizing layer and an absorption axis of the second polarizing layer are parallel to each other. The liquid crystal layer employs a discotic nematic mode.
This arrangement causes the liquid crystal layer in which the liquid crystal molecules have the hybrid alignment to generate a phase difference with respect to linearly-polarized light traveling in an oblique direction from the second polarizing layer toward the liquid crystal layer, so that a polarization state of the linearly-polarized light is changed. Because of this, the linearly-polarized light is absorbed by the first polarizing layer, so that a transmissivity of the linearly-polarized light is reduced.